The poll from the Equality and Human Rights Commission (EHRC) also found more than a third (36%) felt it was reasonable during recruitment to ask women about their plans to have children in the future.
Nearly half (46%) said it was acceptable to try to find out if female job applicants had young children, according to the survey by YouGov for the equalities watchdog.
The EHRC said the figures showed many businesses’ attitudes were “decades behind the law”.
Chief executive Rebecca Hilsenrath said: “It is a depressing reality that, when it comes to the rights of pregnant women and new mothers in the workplace, we are still living in the dark ages.
“We should all know very well that it is against the law not to appoint a woman because she is pregnant or might become pregnant.
“Yet we also know that women routinely get asked questions around family planning in interviews.
“It’s clear that many employers need more support to better understand the basics of discrimination law and the rights of pregnant women and new mothers.”
The report’s figures were drawn from a survey of more than 1,000 “business decision makers” in the private sector.
It also revealed attitudes among many employers about women who they believe “take advantage” of their pregnancy, or are a “burden” to their team, and the belief among some that women should work for an organisation for a year before deciding to have children.
The EHRC is calling on employers to “put a stake in the ground” to try to eliminate such attitudes as well as pregnancy and maternity discrimination by signing up to its Working Forward initiative aimed at improving business practices.
TUC general secretary Frances O’Grady said of the study’s findings: “Thousands are being forced from their job every year. Pregnancy discrimination scars lives and careers.”